Liquid crystal display and  method fo driving thereof

ABSTRACT

A liquid crystal display includes a signal controller having a luminance controller receiving image data from an external graphic source and controlling the luminance of the image data such that the luminance at the gray expressed by a specific data value of the image data is established to be 80 cd/m 2 , and a gamma converter outputting image data each having a gamma characteristic adapted to a gamma 2.2 curve. The gamma converter output is determined without using a look up table based on at least one difference curve having a linear portion, a quartic portion, and a critical value where the linear portion and the quartic portion intersect. The liquid crystal display further includes a data driver receiving the image data for selecting and outputting gray voltages corresponding to the image data, and an inverter controlling a lamp to emit light with a luminance of 80 cd/m 2  or more.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of a co-pending U.S.patent application Ser. No. 10/704,828, filed Nov. 12, 2003, whichclaims priority to and the benefit of Korea Patent Application No.2002-0070050 filed on Nov. 12, 2002 in the Korean Intellectual PropertyOffice, the content of which are all hereby incorporated b reference intheir entirety.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a liquid crystal display and a drivingmethod thereof.

(b) Description of the Related Art

Recently, in the field of a display device such as a personal computerand a television, it is required that the display device should involvea light weight, a thin thickness and a large screen size. In order tofulfill such requirements, a flat panel display such as a liquid crystaldisplay (LCD) has been developed instead of the cathode ray tube, andapplied for practical use in the field of computers, and televisions.

The LCD has a panel with a matrix-typed pixel pattern, and a counterpanel facing the former panel. A liquid crystal material bearing adielectric anisotropy is injected between the two panels. The lighttransmission through the panels is controlled through varying thestrength of the electric fields applied to both ends of the two panels,thereby displaying the desired images.

The display device usually represents original images on the screen byway of the RGB color space intrinsic thereto. That is, when the colorspace is expressed by way of a plurality of gray levels, gammacorrection is made by way of a luminance curve corresponding to eachgray level, that is, by way of a gamma curve. A color correction isadditionally made, thereby recovering the original images. However, asthe RGB color space is mostly device-dependent, the designer of thedisplay device as well as the user thereof should consider the imageprofile intrinsic to the device when the original images arerepresented. This is a considerable burden to them. As the kind and thecharacteristic of the display device are diversified in various manners,it is needed to make a definition of a standard color space for thedisplay device. In this connection, a sRGB color space being the unitstandard RGB color space as the average concept of the RGB monitors wasproposed on November, 1996 by the HP Company and the MS Company. Sincethen, the sRGB color space has been accepted as a standard color spaceon Internet.

A need is made to realize such a sRGB color space with the LCD.

Three requirements should be fulfilled to realize the sRGB color spacewith the LCD. First, the display luminance level with respect to themaximum input gray level should be established to be 80 cd/m². Second,the gamma curve expressing the luminance characteristic of the inputgray level should agree to the gamma 2.2 curve. Third, the display modeloffset with respect to the RGB colors should be established to be zero.

It is required for the LCD to realize such a sRGB color space.

SUMMARY OF THE INVENTION

It is a motivation of the present invention to provide a liquid crystaldisplay which realizes a sRGB color space, and a driving method thereof.

The liquid crystal display includes a signal controller having aluminance controller receiving image data from an external graphicsource and controlling the luminance of the image data such that theluminance at the gray expressed by a specific data value of the imagedata is established to be 80 cd/m², and a gamma converter outputtingimage data each having a gamma characteristic adapted to the gamma 2.2curve. The gamma converter output is determined without using a look uptable based on at least one difference curve, where the at least onedifference curve has a linear portion, a quartic portion, and a criticalvalue where the linear portion and the quartic portion intersect.

The liquid crystal display further includes a data driver receiving theimage data from the signal controller and selecting and outputting grayvoltages corresponding to the image data, and an inverter controlling alamp such that the lamp emits light with a luminance of 80 cd/m² ormore.

With the liquid crystal display, the luminance of a backlight isdetermined to be a specific value larger than 80 cd/m², and theluminance of the input image data is controlled such that the luminancethereof at the specific data value is established to be 80 cd/m².Furthermore, the gamma characteristic of the image data RGB is convertedto be adapted to the gamma 2.2 curve required for the sRGB color space.The gamma converter output is determined without using a look up tablebased on at least one difference curve, and the at least one differencecurve has a linear portion, a quartic portion, and a critical valuewhere the linear portion and the quartic portion intersect. In this way,the sRGB mode is realized with the liquid crystal display, and thedisplay quality of the liquid crystal display can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent by describingembodiments thereof in detail with reference to the accompanyingdrawings in which:

FIG. 1 is a block diagram of an LCD according to an embodiment of thepresent invention;

FIG. 2A is an exemplary graph illustrating luminance of an LCD asfunction of gray;

FIG. 2B shows an exemplary graph illustrating gamma curves of an LCDincluding an original gamma curve and a gamma 2.2 curve for sRGB colorspace;

FIG. 3 is a detailed block diagram of the luminance controller and thegamma converter shown in FIG. 1;

FIG. 4 is a graph showing a gamma 2.2 curve and an original gamma curvefor illustrating the conversion of the gamma curve at the gammaconverter shown in FIG. 3;

FIGS. 5 and 6 are block diagrams of an LCD according to otherembodiments of the present invention;

FIG. 7 is a graph illustrating the gray difference between input(original) image data and corresponding output (target) image data asfunction of the gray of the input image data in an LCD according to anembodiment of the present invention;

FIG. 8 is a flowchart illustrating an exemplary gamma conversion processby way of mathematical operation in an LCD according to an embodiment ofthe present invention; and

FIG. 9 illustrates a method of driving an LCD in a sRGB color spaceaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the inventions are shown.

In the drawings, the thickness of layers and regions are exaggerated forclarity. Like numerals refer to like elements throughout. It will beunderstood that when an element such as a layer, region or substrate isreferred to as being “on” another element, it can be directly on theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly on” another element,there are no intervening elements present.

Now, liquid crystal displays and driving methods thereof according toembodiments of the present invention will be described with reference tothe accompanying drawings.

FIG. 1 is a block diagram of an LCD according to an embodiment of thepresent invention.

As shown in FIG. 1, an LCD according to an embodiment of the presentinvention includes a liquid crystal panel assembly 10, a gate driver 20,a data driver 30, a signal controller 40, a voltage generator 50, a lamp60, and an inverter 70.

The liquid crystal panel assembly 10 includes a plurality of gate lines(not shown) extending in a transverse direction and transmitting gatevoltages, a plurality of data lines (not shown) extending in alongitudinal direction and transmitting data voltages, and a pluralityof pixels (not shown) connected to the gate lines and the data lines andarranged in a matrix. Each pixel includes a liquid crystal capacitor(not shown) and a switching element such as a thin film transistor (TFT)selectively transmitting the data voltages to the liquid crystalcapacitor in response to the gate voltages.

The signal controller 40 receives image data RGB from an externalgraphic source (not shown) together with input control signals such assynchronization signals Hsync and Vsync, a data enable signal DE, and aclock signal MCLK for displaying the image data RGB. The signalcontroller 40 performs luminance control and gamma correction on theimage data RGB to obtain corrected image data R′G′B′, and outputs thecorrected image data R′G′B′ to the data driver 30. Furthermore, thesignal controller 40 generates control signals such as a horizontalclock signal HCLK, a horizontal synchronization start signal STH, a loadsignal LOAD, a gate clock signal Gate clock, a vertical synchronizationstart signal STV, and an output enable signal OE for controlling thedisplay operations of the gate driver 20 and the data driver 30, andoutputs them to the relevant drivers 20 and 30.

The signal controller 40 includes a control signal processing block 41and a data processing block including a luminance controller 42, and thegamma converter 43.

The control signal processing block 41 generates the control signalsHCLK, STH, LOAD, Gate clock, STV and OE based on the synchronizationsignals Hsync and Vsync, the data enable signal DE, and the clock signalMCLK.

The luminance controller 42 controls the luminance of the image data RGBsuch that the luminance represented by a predetermined gray value (ordata value) of the image data RGB be about 80 cd/m². The luminancecontrol of the luminance controller 42 is described with reference toFIG. 2A, which is an exemplary graph illustrating luminance of an LCD asfunction of gray.

As shown from a solid curve in FIG. 2A, it is assumed that the luminancerepresented by a predetermined gray D is smaller than 80 cd/m², whilethe gray D+ΔD represent the luminance of 80 cd/m². The luminancecontroller 42 performs the luminance control by adding the value AD intoinput image data such that the luminance represented by the gray D ofthe predetermined input image data reach 80 cd/m². Therefore, theluminance control moves the solid luminance curve in a direction A to adotted luminance curve. The predetermined gray D is preferably a maximumgray.

The gamma converter 43 converts a gamma characteristic of the image datafrom the luminance controller 42 such that it is adapted to a gamma 2.2curve, and it outputs the converted image data R′G′B′ to the data driver30. The gamma converter 43 may perform the gamma conversion by way of alook-up table (LUT) or a mathematical operation realized on anapplication specific integrated circuit (ASIC). The configuration shownin FIG. 1 is obtained when using a look-up table. In this case, thelook-up table includes a mapping from the original (input) image dataRGB to the converted (output) image data R′G′B′. The gamma converter 43retrieves a converted data corresponding to an input image data from thelook-up table, and it output the converted image data. Although FIG. 1shows that the bit number (n bits) of the converted image data R′G′B′ isequal to the bit number (n bits) of the original image data RGB, the bitnumber of the converted image data RGB may be larger than the bit numberof the original image data RGB in order to enhance the precision of thegamma conversion.

FIG. 2B shows an exemplary graph illustrating gamma curves of an LCDincluding an original gamma curve and a gamma 2.2 curve for a standardRGB (sRGB) color space. In the figure, a horizontal axis indicates anormalized input gray level while a vertical axis indicates a normalizedluminance.

The data driver 30 receives and stores the converted image data R′G′B′from the gamma converter 43 of the signal controller 40 insynchronization with the control signals HCLK and STH. The data driver30 receives a plurality of gray voltages Vgray, which are analogvoltages to be actually applied to the liquid crystal panel assembly 10,from the voltage generator 50. The data driver 30 selects the grayvoltages Vgray corresponding to the image data R′G′B′ for the respectivepixels, and outputs the selected gray voltages as the data voltages tothe liquid crystal panel assembly 10 in response to the load signalLOAD.

The gate driver 20 receives the gate clock signal Gate clock, the outputenable signal OE, and the vertical synchronization start signal STV fromthe signal controller 40, and it also receives gate voltages Vgate fromthe voltage generator 50. The gate driver 20 sequentially outputs thegate voltages for selecting the gate lines on the liquid crystal panelassembly 10 in accordance with the output enable signal OE and the gateclock signal Gate clock, thereby sequentially scanning the gate lines onthe liquid crystal panel assembly 10.

The lamp 60 and the inverter 70 form a backlight for the liquid crystalpanel assembly 10, and the inverter 70 controls the light emission ofthe lamp 60. In this embodiment, it is established that the inverter 70controls the lamp 60 with a luminance of 80 cd/m2 or more to fulfill theluminance requirement of the sRGB color space.

When a gate line is selected by the gate voltages Vgate, the pixelsconnected to the gate line become in a write-enable state to be appliedwith the data voltages through the data lines. The pixels displaypredetermined luminance levels corresponding to the data voltages and adesired image is displayed on an entire screen in such a way.

The operation of the gamma converter 43 will be now described more indetail with reference to FIGS. 3 and 4.

FIG. 3 is a detailed block diagram of the luminance controller 42 andthe gamma converter 43 shown in FIG. 1, and FIG. 4 is a graph showing agamma 2.2 curve and an original gamma curve for illustrating theconversion of the gamma curve at the gamma converter 43 shown in FIG. 3.

As shown in FIG. 3, the gamma converter 43 includes an R data modifier431, a G data modifier 432, and a B data modifier 433. The datamodifiers 431-433 perform the conversion of the gamma characteristics inrelation to the respective RGB colors.

More specifically, each data modifier 431-433 maps an input image datarepresenting a luminance level on the gamma 2.2 curve into an outputimage data representing the same luminance level on the original gammacurve. As shown in FIG. 4, it is assumed that the gray level of theinput image data is 128. The luminance of the 128-th gray level on theoriginal gamma curve is different from the luminance of the 128-th graylevel on the gamma 2.2 curve. Instead, the 129.4-th gray level on theoriginal gamma curve represents the same luminance as the 128-th graylevel on the gamma 2.2 curve. Each data modifier 431-433 maps the inputimage data with the 128-th gray level into the output image data withthe 129.4-th gray level. For this purpose, each data modifier 431-433includes a look-up table including a map between gray levels on thegamma 2.2 curve and gray levels on the original gamma curve, whichrepresent equal luminance. The look-up tables for the data modifiers431-433 may be implemented in respective non-volatile memories such asROM (read only memory) or implemented in one ROM. hi order to enhancethe precision of the gamma conversion, the bit number of the outputimage data is larger than that of the input image data such thatdecimals under the decimal point of the gray levels as shown in FIG. 4can be expressed.

FIGS. 5 and 6 are block diagrams of an LCD according to otherembodiments of the present invention.

The LCD shown in FIG. 5 further includes a ROM controller 44 and anexternal target image data storage 45 in addition to a gamma converter43. The gamma converter 43 includes R, G and B data modifiers 431-433,each including a volatile memory such as a random access memory (RAM).

The external target image data storage 45 stores a look-up tableincluding a map between gray levels on the gamma 2.2 curve and graylevels on the original gamma curve for each color, which represent equalluminance. The ROM controller 44 loads the look-up table in the storage45 into the R, G and B data modifiers 431-433. Since the otheroperations are similar to those shown in FIG. 3, the description thereofis omitted here.

Since the look-up table is stored in the external storage 45, thisembodiment easily copes with the alteration of the panel assembly 10without changing the gamma converter 43.

The LCD shown in FIG. 6 further includes an internal target image datastorage 46 as well as a ROM controller 44, an external target image datastorage 45 in addition to a gamma converter 43 as compared with the LCDshown in FIG. 5. The gamma converter 43 also includes R, G and B datamodifiers 431-433, each including a volatile memory such as a randomaccess memory (RAM).

Like the external target image data storage 45, the internal targetimage data storage 46 stores a look-up table including theabove-described map. The ROM controller 44 loads the look-up tablestored in the external storage 45 or in the internal storage 46 into theR, G and B data modifiers 431-433. Other operations are similar to thoseshown in FIG. 3, and hence, description thereof will be omitted here.

Now, gamma conversion by way of a mathematical operation according to anembodiment of the present invention will be described with reference toFIGS. 7 and 8.

FIG. 7 is a graph illustrating the gray difference between input(original) image data and corresponding output (target) image data asfunction of the gray of the input image data in an LCD according to anembodiment of the present invention, and FIG. 8 is a flowchartillustrating an exemplary gamma conversion process by way ofmathematical operation in an LCD according to an embodiment of thepresent invention.

It is assumed that the image data RGB are 8 bit signals capable ofrepresenting 256 grays.

As shown in FIG. 7, there is no gray difference between the target imagedata and the original image data for green image data G, while curvesillustrating the gray difference between the target image data and theoriginal image data for red and blue image data R and B change theirshape near the gray level of 160. The gray difference AR and AB betweenthe original data and the target data for red and blue image data R andB can be approximately expressed by:

$\begin{matrix}\begin{matrix}{{\Delta \; 6} = {6 - \frac{6 \times \left( {160 - R} \right)}{160}}} & {{{{{if}\mspace{14mu} R} < 160},\mspace{14mu} {and}}} \\{{6 - \frac{6 \times \left( {R - 160} \right)^{4}}{\left( {255 - 160} \right)}}} & {{{{{if}\mspace{14mu} R} \geq 160},\mspace{14mu} {and}}}\end{matrix} & (1) \\\begin{matrix}{{\Delta \; B} = {{- 6} + \frac{6 \times \left( {160 - B} \right)}{160}}} & {{{{{if}\mspace{14mu} R} < 160},\mspace{14mu} {and}}} \\{\frac{{- 6} + {6 \times \left( {B - 160} \right)^{4}}}{\left( {255 - 160} \right)^{4}}} & {{60,\mspace{14mu} {and}}}\end{matrix} & (2)\end{matrix}$

where R and B are the grays of the original data for red and blue imagedata, respectively. As shown in FIG. 7, the gray difference curves ΔRand ΔB are symmetrical and have values less than the critical value thatare shown by a linear portion of their respective difference curve.Further, the gray difference curves ΔR and ΔB have values greater thanthe critical value that are shown by a quartic (4th power) portion oftheir respective difference curve. The linear and quartic portions ofeach curve intersect at the critical value.

First, as shown in FIG. 8, when an 8 bit red image data are input, it isdetermined whether the gray R of the input data is larger than acritical value of “160” (S501).

When the input gray R is larger than the critical value, the criticalvalue is subtracted from the input gray (S502). Then, the resultantvalue (R−160) may be multiplied by 1/(255−160). However, since1/(255−160) is roughly approximated to 11/1024(=2¹⁰), for the purpose ofsimplification, (R−160) is multiplied by 11 and the lower 10 bits arerounded off (S503). Thereafter, (R−160)×11/1024 may be squared twice ina sequential manner. These operations can be made by way of a pipelineon ASIC (S504, S505). The resultant value of ((R−160)×11/1024)⁴ ismultiplied by 6 (S506) and the resultant value of 6×(((R−160)×11/1024)⁴)is subtracted from 6, thereby obtaining the value of AR in accordancewith Relation 1 (S507).

When the input gray R is smaller than the critical value in the step501, the input gray R are subtracted from the critical value (S511).Then, the resultant value (160−R) may be multiplied by 1/160. However,since 1/160 is roughly approximated to 13/2048(=2¹¹), (160−R) ismultiplied by 13 and then the lower 11 bits are rounded off (S512).Thereafter, (160−R)×13/2048 is multiplied by 6 (S513). The resultantvalue of ((160−R)×13/204 8)×6 from the step S513 is subtracted from 6,thereby obtaining the value of AR in accordance with Relation 1 (S514).

In order to get 10 bit output data from AR obtained at the step S507 orS514, the 8 bit input data is multiplied by “4” to be converted into 10bit data and is added to the calculated value ΔR (S508).

Similarly, blue output image data B′ can be calculated based on Relation2.

The gamma conversion by way of a mathematical operation does not requirea memory for storing a look-up table. The storage capacity of ROM or RAMrequired for storing the lookup table is considerably great. Forinstance, the storage capacity of 6144 (3×256×8) bits are required for 8bit image data. Accordingly, the gamma conversion according to thisembodiment removes a large amount of storage capacity and reduces thepower consumption due to the memory.

A method of driving an LCD according to an embodiment of the presentinvention will be now described with reference to FIG. 9.

FIG. 9 illustrates a method of driving an LCD in a sRGB color spaceaccording to an embodiment of the present invention.

As shown in FIG. 9, a method of driving an LCD including a backlightunit according to an embodiment of the present invention includes afirst step for controlling the backlight and a second step for gammacorrection. The backlight unit includes at least one lamp and aninverter for controlling the lamp.

In the first step, the inverter is controlled such that the lamp emitslight with a luminance equal to or larger than 80 cd/m², which isrequired for the sRGB color space.

The second step includes the substeps of luminance control and gammaconversion as described above, hi detail, the luminance of the imagedata is controlled such that the luminance level represented by apredetermined gray level of image data be 80 cd/m², and the gammacharacteristic of the input image data are converted to be adapted tothe gamma 2.2 curve.

As described above, the luminance of the backlight is determined to be aspecific value larger than 80 cd/m², and the luminance of the image datais controlled such that the luminance of the input image data satisfies80 cd/m² at the specific image data value. In this way, the sRGB modecan be realized with the LCD, and the display quality of the LCD can beimproved.

While the present invention has been described in detail with referenceto the embodiments, those skilled in the art will appreciate thatvarious modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. A display comprising: a signal controller including a luminancecontroller processing input image data with respective grays such thatthe luminance represented by a predetermined gray of the input imagedata be about 80 cd/m2, and a gamma converter outputting output imagedata have gamma characteristic adapted to a gamma 2.2 curve based oninput image data; a data driver selecting and outputting gray voltagescorresponding to the image data from the signal controller; and aninverter controlling a lamp such that the lamp emits light withluminance equal to or larger than 80 cd/m2.
 2. The liquid crystaldisplay of claim 1, wherein the processing of the luminance controllerincludes addition of a predetermined data value to the input image datasuch that the luminance represented by the predetermined gray of theinput image data be 80 cd/m2.
 3. The liquid crystal display of claim 1,wherein the gamma converter comprises an R data modifier, a G datamodifier and a B data modifier for performing the gamma conversion forthe input image data for respective red, green and blue colors, and eachof the data modifiers maps the input image data into output image datahaving a gamma characteristic adapted to the gamma 2.2 curve.
 4. Theliquid crystal display of claim 3, wherein the data modifiers include anonvolatile memory.
 5. The liquid crystal display of claim 1, whereinthe gamma converter comprises an R data modifier, a G data modifier anda B data modifier for performing the gamma conversion for the inputimage data for respective red, green and blue colors, the liquid crystaldisplay further comprises a target image data storage storing a map fromthe input image data into output image data having a gammacharacteristic adapted to the gamma 2.2 curve and a controller loadingthe map stored in the target image data storage into the data modifiers,and the data modifiers select the output image data corresponding to theinput image data from the loaded map and outputting the selected outputimage data.
 6. The liquid crystal display of claim 5, wherein the datamodifiers comprise a volatile memory, and the target image data storagecomprises a nonvolatile memory element.
 7. The liquid crystal display ofclaim 5, wherein the target image data storage includes a nonvolatilememory in the signal controller and a nonvolatile memory elementprovided external to the signal controller.
 8. The liquid crystaldisplay of claim 1, wherein the gamma converter obtains the output imagedata from the input image data using a look up table.
 9. A method ofdriving a display, the method comprising: controlling luminance of abacklight to be larger than about 80 cd/m2; controlling luminance ofimage data with respective grays such that the luminance levelrepresented by a predetermined gray of input image data be about 80cd/m2; and converting gamma characteristic of the input image data to beadapted to a gamma 2.2 curve.
 10. The method of claim 9, wherein thegamma characteristic conversion includes a mapping operation using a mapstored in a nonvolatile memory.